1. Field of the Invention
The present invention relates to a light source device and more particularly to the techniques to be effectively applied to a light source device of the type used in a liquid crystal display apparatus which requires a flat plane type light source.
2. Description of the Prior Art
An example of the flat plane type light source device of the prior art is described in the U.S. Pat. No. 4,059,916. As shown in FIG. 8, a light source 4B is arranged at the one end part of a light guiding material 4A, the light guiding material 4A is formed by a material which efficiently transmits the light, the one surface 4a in the observing side of which is formed as a smooth surface, while the other surface 4b is provided as a rough surface. The rough surface 4b diffuses the light emitted from the light source 4B and reflects the light emitted from the light source 4B to the smooth surface 4a. The light guiding material 4A is formed as shown with a sloping plane so that it becomes thinner as it extends from the light source 4B.
In the prior art, the transparent board gradually becomes thinner, as shown in FIG. 8, as the coordinate X increases and a flat plane type light source is attained by providing a light source 4B, for example, a fluorescent lamp, at the one end face of the light guiding element 4A. Therefore, (a) a single Unit of fIuorescent lamp is used, as shown in FIG. 8, for a flat plane type light source of comparatively small area, and (b) at pair of fluorescent lamps are provided vertically at the end face, as seen in FIG. 9, in order to attain a large area and high brightness flat plane type light source. However, in the prior art, the light source (a) is thin and light weight but is not suited for high brightness and large size display, while the light source (b) provides high brightness and large size display but is difficult to realize as a thin and light-weight display apparatus. A light source of this kind is also disclosed, for example, in U.S. Pat. No. 4,487,481.
A flat plane type light source of another type comprising a light guiding material is disclosed in International Publication Number W083/03013. This example is shown in FIG. 10 and FIG. 11. FIG. 10 is a disassembled perspective view of the display section and light source section, while FIG. 11 shows the unit assembled as a display. In FIG. 10, 4A designates a light guiding material and the end parts thereof are provided with a holes 15 formed to accommodate point type light sources 14B. This light guiding material has a flat light emitting side 4a and becomes thinner as it goes near the center from the end part. But the curved surface 4b to determine the thickness of the light guiding material is not disclosed. Moreover, the surface treatment for the light guiding material is not disclosed. A similar light guiding material is also disclosed in U.S. Pat. No. 4,641,925, but in this case, there is no description as to the manner of changing the thickness of the light guiding material or the surface treatment thereof. The light sources disclosed in the patents listed above may be considered to be used for only a small area display, judging from the objects thereof.